Re: [eigen] about JacobiSVD's options

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

my paste was missing the last line, here it is complete:

=== 23:23:18 ~$ g++ a.cpp -o a -I eigen2 -O2 && time g++ a.cpp -o a -I
eigen2 -O2 -DSVDOPTIONS=Square && ls -l a

real    0m2.460s
user    0m2.348s
sys     0m0.108s
-rwxr-xr-x 1 bjacob users 51007 sept. 15 23:23 a
=== 23:23:29 ~$ g++ a.cpp -o a -I eigen2 -O2 && time g++ a.cpp -o a -I
eigen2 -O2 && ls -l a

real    0m6.530s
user    0m6.372s
sys     0m0.152s
-rwxr-xr-x 1 bjacob users 142180 sept. 15 23:23 a

Benoit

2009/9/15 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>:
> Hi,
>
> 2009/9/15 Gael Guennebaud <gael.guennebaud@xxxxxxxxx>:
>> hi,
>>
>> the "Options" template parameter of JacobiSVD seems to be over-complicated.
>>
>> What about removing the Square/MoreColsThanRows/etc. stuff from the
>> Option template parameter ? For small fixed sizes they can be
>> automatically extracted at compile time, and for dynamic sizes .. well
>> who care ? Moreover in the worst case, if the user use various matrix
>> shapes he can generate 3 instances of JacobiSVD for the same matrix
>> type...
>
> The reason for making these template parametes is to avoid compiling
> useless code: so it allows to compile much faster, and produce much
> smaller code.
>
> When you give JacobiSVD a rectangular matrix, it does a full-pivoting
> QR to reduce to the case of a square matrix. This step has to be done
> differently depending on whether m>=n or m<=n. I already have made
> sure that these 2 paths use the same instantiation of
> FullPivotingHouseholderQR, so it is only instantiated once. Still,
> that instantiation is relatively expensive (as is any householder QR
> --- the full pivoting version is only marginally more expensive to
> compile).
>
> That's why I provided the user a way to avoid compiling this code if
> it's not going to be used. By default the code is compiled and the
> user doesn't need to care about anything, just use
> JacobiSVD<MatrixXf>, but an advanced user may be interested in that.
>
> Here is a benchmark for compilation times and code size. (attached a.cpp)
>
> Result:
> === 23:23:18 ~$ g++ a.cpp -o a -I eigen2 -O2 && time g++ a.cpp -o a -I
> eigen2 -O2 -DSVDOPTIONS=Square && ls -l a
>
> real    0m2.460s
> user    0m2.348s
> sys     0m0.108s
> -rwxr-xr-x 1 bjacob users 51007 sept. 15 23:23 a
>
> === 23:23:29 ~$ g++ a.cpp -o a -I eigen2 -O2 && time g++ a.cpp -o a -I
> eigen2 -O2 && ls -l a
>
> real    0m6.530s
> user    0m6.372s
> sys     0m0.152s
>
> So, passing the Square template parameter almost divides compilation
> times and code size by 3.
>
> (g++ 4.4.1 x86)
>
>>
>> Also, I'm wondering whether the ComputeU and ComputeV parameters could
>> be runtime parameters ? I doubt there is any performance difference,
>> but of course to be sure we should bench it. If I have time I'll do
>> it.
>
> Hm, that makes more sense indeed, we need to bench, not only
> performance but code size and compilation times.
> At the same time, making them runtime params is good in itself as it
> can mean fewer template instantiations, precisely.
>
> Benoit
>